The invention is in the field of offset lithography, and more specifically relates to a lithographic dampening solution and a method for dampening a lithographic plate.
Offset lithography is a printing process in which an image carrier takes the form of a plate which includes a photochemically produced ink-receptive image area and an ink-repellant non-image area. Inks used in traditional offset lithography processes are oil-based, and thus the image area on a printing plate typically is hydrophobic and the non-image area is hydrophilic. In an offset lithographic operation, the plate is mounted on a cylinder and is dampened with a dampening solution, which is typically an aqueous solution of chemicals, and which adheres to the non-image area of the plate. The plate is next contacted by inked rollers, which apply ink to the image area of the plate. The inked image is transferred to a rubber-like blanket, on which the image becomes reversed. The inked image on the blanket is then transferred to a printing substrate, typically a sheet of paper, thereby producing an impression of the inked image on the paper.
The dampening solution is used to cause a crisp division between the image and non-image areas of the printing plate, to thereby provide high resolution in the print. Conventional dampening solutions include water and a gum and, typically, an acid or a base, corrosion inhibitor, buffer, wetting agent, drying stimulator, fungicide, anti-foaming agent, and possibly other ingredients of a conventional nature. The gum is used in an amount effective to provide sufficient film formability to enhance the ink repellency of the non-image area of the lithographic plate.
Historically and conventionally, the gum used in dampening solutions is gum arabic, as disclosed, for instance, in U.S. Pat. Nos. 4,030,417; 4,150,996; 5,256,190; and 5,382,298; in EP 066176,249752; and 251,621; and in DE 3,536,485. Although dampening solutions made with gum arabic are satisfactory from a technical standpoint, there are a number of potential and actual problems in obtaining gum arabic. Gum arabic is the exudate of a woody plant (acacia) that is grown in areas of the world that presently are politically unstable, for example, the Sudan. For this reason, and because the supply of gum arabic is largely dependent upon unpredictable climactic conditions, the constancy of the supply of gum arabic is somewhat unstable. In addition, the quality of harvested gum arabic can be variable.
In recognition of these potential and actual problems, the prior art has provided dampening solutions that include a number of chemically synthesized substitutes for gum arabic. For instance, U.S. Pat. No. 5,279,648 discloses a dampening solution that includes hydroxypropyl cellulose. Another reference, DE 2,504,594, discloses a dampening solution that includes an acrylamide-acrylic acid copolymer. These dampening solutions are believed to be somewhat unsatisfactory. A dampening solution that includes a chemically derivatized dextran is taught in EP 517,959, and a dampening solution that includes a chemically derivatized pullulan is disclosed in DE 2,648,805. While these solutions are somewhat satisfactory, the need for derivatization adds cost to the formulation process for these solutions.
It is a general object of the invention to provide a dampening solution that is composed of water and a naturally occurring biopolymer. A related general object of the invention is to provide a method for dampening a lithographic plate.
It has now been found that aqueous solutions of hemicellulose, particularly corn-hull-derived hemicellulose, may be used to dampen lithographic plates. Hemicellulose is a naturally occurring component of seed hulls, such as corn seed hulls, and is obtained from the hulls during industrial processing. It has further been found that the enzyme digestion product of hemicellulose, in particular, the xylanase digestion product, may be used in a dampening solution to dampen a lithographic plate. The commercial availability of hemicellulose is more steady and reliable than that of gum arabic, in that the corn crop in the United States is large and stable, and the hemicellulose content of corn seeds is uniform and predictable. For this reason, the use of hemicellulose as a dampening solution gum is advantageous. Moreover, although the use of soybean hemicellulose has been provided in the art (for instance in U.S. Pat. No. 5,615,613), the composition of soybean derived hemicellulose is substantially similar to that of gum arabic, while the composition of corn hull hemicellulose is markedly different. The ability of corn hull derived hemicellulose to function in a dampening solution in a manner as satisfactory as gum arabic is surprising, in light of the substantial differences in composition between corn hull derived hemicellulose and gum arabic.
In one embodiment, the invention generally provides a method for dampening a lithographic plate. The method generally comprises the step of applying a dampening solution to at least the non-image area of the lithographic plate. The dampening solution comprises an aqueous solution of a corn-hull-derived hemicellulose, which is present in the dampening solution in an amount effective to provide sufficient film formability to enhance the ink repellency of the non-image area of the plate. In another embodiment, the method includes the steps of applying a dampening solution that includes water and the enzyme digestion product of a hemicellulose, such as a corn hull derived hemicellulose, or alternatively a hemicellulose derived from another seed source. The invention further provides a dampening solution that includes the enzyme digestion product of a hemicellulose and a surfactant, the surfactant being present in an amount effective to enhance the wettability of the dampening solution.
Other features and embodiments of the invention are set forth in the following description of the preferred embodiment, and in the appended claims.
The invention generally contemplates a dampening solution that includes hemicellulose, in particular, corn-hull-derived hemicellulose, or that includes the enzyme digestion product of hemicellulose. Hemicellulose is a water-soluble, highly branched polymer of xylose having side-chains composed of arabinose, galactose, and terminal glucuronic acid. The isolation of corn hull hemicellulose from corn hulls is taught in U.S. Pat. Nos. 2,801,955; 2,868,778; 3,716,526; and 4,038,481.
Once isolated, the hemicellulose may be used in an aqueous solution as a dampening solution, or the hemicellulose may be treated with an enzyme to thereby result in an enzyme digestion product that may be used in aqueous solution as a dampening solution. Most preferably, the enzyme is a xylanase enzyme. By xe2x80x9cdigestionxe2x80x9d is contemplated the full or partial, most preferably, partial, digestion by the enzyme to yield a product that is fully or partially depolymerized by the enzyme, and in the case of partial digestion, possibly including some undigested hemicellulose. The hemicellulose used in conjunction with this embodiment of the invention preferably is corn-hull-derived hemicellulose, but alternatively may be a hemicellulose derived from another source, such as rice, wheat, or soybeans. While it is feasible to use hemicellulose without enzymatic digestion, such is less preferred inasmuch as the dampening solution generally will not be satisfactory for commercial purposes. In accordance with the invention, the enzyme digestion should be sufficient to enhance the ability of the hemicellulose to repel ink in the dampening solution. While it is not intended to limit the invention to a particular theory of operation, it is believed that the enzyme will reduce the average molecular size of the xylose polymers in hemicellulose to thereby cause the hemicellulose digestion product to interact more successfully with the surface of the lithographic printing plate to form a hydrophilic surface which repels hydrophobic lithographic ink.
After enzyme treatment, the digest product may be further treated with a bleach, which preferably is hydrogen peroxide. The hydrogen peroxide is believed to further reduce the molecular weight of the hemicellulose and to improve the color of the product. The bleach alternatively may comprise another oxidant, such as sodium hypochlorite. When the dampening solution includes undigested hemicellulose, such undigested hemicellulose similarly may be treated with bleach. Details concerning treatment of hemicellulose with hydrogen peroxide are taught in WO 98/40413.
In accordance with the invention, a dampening solution that generally includes water and one or more gums is provided. The gum may be corn-hull-derived hemicellulose, or the enzyme digestion product of a hemicellulose, alone or in combination with one or more additional gums. The additional gum may be a natural, semi-synthetic, or synthetic gum. Examples of conventional naturally occurring gums include gum arabic, tragacanth gum, carageenan, xanthan gum, gelatin, casein sodium, guar gum, gum tare, glue plants (funori), agar, furcellaran, tamarind seed polysaccharides, gumkaraya, hibiscus, pectin, sodium alginate, pullulan, gellan gum, locust bean gum, albumin, and various starches. Examples of semi-synthetic gums include carboxymethyl cellulose (CMC), methyl cellulose (MC), ethyl cellulose (EC), hydroxyethyl cellulose (HBC), alginic acid propylene glycol ester and chemically modified starches including soluble starches. Examples of conventional synthetic gums include polyethylene glycol and copolymers thereof, polyvinyl alcohol and copolymers thereof, polyvinyl pyrrolidone, polyacrylamide and copolymers thereof, polyacrylic acid and copolymers thereof, vinyl methyl ether/maleic anhydride copolymer, vinyl acetate/maleic anhydride copolymer and polystyrenesulfonic acid and copolymers thereof.
The gums should be present in the dampening solution in a total amount ranging from about 0.1% to about 35% by weight; most preferably in an amount ranging from about 0.1% to about 5.0% by weight, these percentages referring to the amount of gum that remains in solution should any precipitate. The pH of the dampening solution preferably ranges from about 3.5 to about 5.5. To adjust the pH to within this range, an acid typically will be added during preparation of the dampening solution. It is suitable to use a mineral acid, an organic acid, an inorganic salt, or the like to adjust the pH to the desired range. Examples of mineral acids include sulfuric acid, nitric acid, phosphoric acid, and metaphosphoric acid. Examples of organic acid include lactic acid, citric acid, oxalic acid, malonic acid, p-toluene sulfonic acid, tartaric acid, malic acid, lebulinic acid, ascorbic acid, gluconic acid, hydroxyacidic acid, acitic acid, sulfanic acid, phytic acid, and the like. Alkali metal salts, alkaline earth metal salts, or ammonium salts of these mineral acids and organic acids also may be used. The foregoing acids and salts may be used alone or in a combination of two or more of them.
The dampening solution preferably includes a wetting agent, such as a surfactant, which should be used in an amount effective to enhance the wettability of the dampening solution. Preferably, the wetting agent is used in an amount ranging from about 0.5% to about 6%; but more preferably, the wetting agent is used in an amount ranging from about 0.35% to about 3.5% by weight. One wetting agent or a combination of wetting agents may be used.
The surfactant may be an anionic, nonionic, cationic, or amphoteric surfactant. Examples of suitable anionic surfactants include salts of aliphatic alcohol sulfates, salts of aliphatic alcohol phosphates, salts of dibasic fatty acid ester sulfonates, salts of fatty acid amide sulfonates, salts of alkylaryl sulfonates and salts of naphalene sulfonate condensed with formaldehyde. Salts of fatty acids, salts of abletic acid, salts of alkanesulfonic acids, salts of hydroxyalkanesulfonic acids, salts of dialkylsulfonosuccunic acids, salts of straight-chain alkylbenzenesulfonic acids, salts of branched alkylbenzenesulfonic acids, salts of alkylnaphthalenesulfonic acids, salts of alkylphenoxypolyoxyethylenepropylsulfonic acids, salts of polyoxyethylene alkylsulfophenyl ethers, sodium salt of N-methyl-N-oleyltaurine, disodium salt of N-alkylsulfosuccinic acid monoamides, salts of petroleum sulfonic acids, sulfated castor oil, sulfated beef tallow, salts of sulfates of fatty acid alkyl esters, salts of alkyl sulfates, salts of sulfates of polyoxyethylene alkyl ethers, salts of fatty acid monoglyceride sulfates, salts of sulfates of polyoxyethylene alkylphenyl ethers, salts of sulfates of polyoxyethylene styrylphenyl ether, salts of alkylphosphoric acids, salts of phosphates of polyoxyethylene alkylphenyl ethers, partial saponification products of styrene/maleic anhydride copolymer, partial sapnification products of olefin/maleic anhydride copolymers and condensates of salts of naphthalenesulfonic acid with formalin. Among them, salts of dialkylsulfonosuccinic acids, salts of alkyl sulfates and salts of alkylnaphthalenesulfonic acids are particularly preferred.
Examples of suitable nonionic surfactant include polyoxyethylene alkyl ethers, polyoxyethylene alkylphenyl ethers, polyoxyethylene polystyrylphenyl ether, polyoxyethylene polyoxypropylene alkyl ether, partial esters of glycerin with fatty acids, partial esters of sorbitan with fatty acids, partial esters of pentaerythritol with fatty acids, esters of propylene glycol with monofatty acids, partial esters of sucrose with fatty acids, partial esters of polyoxyethylene sorbitan with fatty acids, partial esters of polyoxyethylene sorbitol with fatty acids, esters of polyoxyethylene glycol with fatty acids, partial esters of polyglycerine with fatty acids, polyoxyethylenated castor oil, partial esters of polyoxyethylene glycerin with fatty acids, fatty acid diethanolamides, N,N-bis-2-hydroxyalkylamines, polyoxyethylenealkylamines, esters of triethanolamine with fatty acids and triallylamine oxides. Among them, polyoxyethylene alkylphenyl ethers, polyoxylpropylene block polymers, etc., are particularly preferred. Examples of suitable cationic surfactants include alkylamine salts, quaternary ammonium salts, polyoxethylene alkylamine salts, and polyethylene polyamine derivatives.
Further suitable wetting agents include polyols, glycol ethers, and alcohols. The polyol and glycol ether may comprise 2-ethyl-1,3-hexanediol, hexyl carbitol, ethylene glycol, diethylene glycol, triethylene glycol, propylene glycol, dipropylene glycol, tripropylene glycol, hexylene glycol, tetraethylene glycol, 1,5-pentanediol, hexyl cellosolve, glycerin, diglycerin, ethylene glycol monomethyl ether, diethylene glycol monomethyl ether, triethylene glycol monomethyl ether, polyethylene glycol monomethyl ether, propylene glycol monomethyl ether, dipropylene glycol monomethyl ether, ethylene glycolmonopropyl ether, diethylene glycol monopropyl ether, propylene glycol monopropyl ether, dipropylene glycol monopropyl ether, ethylene glycol monoisopropyl ether, diethylene glycol monoisopropyl ether, ethylene glycol monobutyl ether, diethylene glycol monobutyl ether, triethylene glycol monobutyl ether, propylene glycol monobutyl ether, dipropylene glycol monobutyl ether, polypropylene glycol (molecular weight: 200 to 10000), ethylene glycol monoisobutyl ether, diethylene glycol monoisobutyl ether, ethylene glycol monoallyl ether, ethylene glycol monophenyl ether, diethylene glycol monophenyl ether, ethylene oxide adduct of 2-ethyl-1,3-hexanediol, acetylene glycol and ethylene oxide adduct thereof. The alcohol may be, for example, ethyl, n-propyl, isopropyl, n-butyl, isobutyl, n-amyl, or benzyl alcohol.
The dampening solution further may include a preservative, which may be present in any amount effective to provide a preservative effect, i.e., to retard the growth of at least one microorganism such as bacteria, fungi, yeast, and the like. Although the amount of preservative used may depend upon the type of microorganism whose growth is sought to be retarded and the specific chemical employed, the amount of preservative generally should be in the amount of 0.05 to about 1.0% by weight of the dampening solution, more preferably from about 0.1 to about 0.2%. Specific examples of the preservative usable in the present invention include phenol and derivatives thereof, imidazole derivatives, formalin, sodium dehydroacetate, 4-isothiazolin-3-one derivatives, benzotriazole derivatives, amidine guanidine derivatives, quaternary ammonium salts, derivatives of pyridine, guinoline and guanidine, diazine, triazole derivatives, oxazole and oxazine derivatives.
The dampening solution further may include a pH buffer to maintain the pH within the desired range. The pH buffer may be present in any amount effective for this purpose and, preferably, the buffer is present in an amount ranging from about 0.005 to about 2% by weight in general, more preferably, from about 0.001 to about 1% by weight. Examples of pH buffers include alkaline metal oxides, alkaline metal phosphates, alkaline metal carbonates, and silicates.
The dampening solution further may include an anti-foaming agent, which may be present in any amount effective to retard foaming of the composition. Most preferably, the anti-foaming agent is a silicone compound, which may be either an emulsion-type or a xe2x80x9cone-packxe2x80x9d type. The anti-foaming agent may be used in any amount ranging from about 0.005% to about 0.4% by weight, more preferably, from about 0.001% to about 0.3% by weight.
Another optional component of the dampening solution is a chelate compound, which may be added to neutralize calcium and other ions found in the source of the solvent water. The chelate compound may be added in any amounts suitable to neutralize ions present in the solvent water source, and most preferably the chelate compound is present in an amount ranging from about 0.005% to about 6% by weight, more preferably, in an amount ranging from about 0.005 to about 1% by weight. Specific examples of the chelate compound include organic phosphoric acids or phosphonoalkanetricarboxylic acids, such as ethylenediaminetetraacetic acid, the potassium salt of ethylenediaminetetraacetic acid, sodium salt of ethylenediaminetetraaccetic acid, diethylenetriaminepentaacetic acid, the potassium salt of diethylenetriaminepentaacetic acid, sodium salt of diethylenetriaminepentaacetic acid, triethylenetetraminehexaacitic acid, sodium salt of triethylenetetraminehexaacetic acid, hydroxyethylenediaminetriaacetic acid, potassium salt of hydroxyethylethylenediaminetriacetic acid, nitrilotriacetic acid, sodium salt of nitrilotriacetic acid, 1-hydroxyethane-1,1-diphosphonic acid, potassium salt of 1-hydroxyethane-1,1-diphosphonic acid, sodium salt of 1-hydroxyethane-1,1-diphosphonic acid, sodium salt of 1-hydroxyethane-1,1-diphosphonic acid, aminotri(methylenephosphonic acid), potassium salt of aminotri(methylenephosphonic acid) and sodium salt of aminotri(methylenephosphonic acid). Further, it is also possible to use organic amine salts of the chelate compounds instead of the potassium salt and sodium salt of the chelate compounds.
The dampening solution further may optionally include a corrosion inhibitor, which may be present in any amount effective to prevent metal oxidation. Examples include benzotriazole, tolyltriazole, benzoamidazole and 2-mercaptobenzoimidazole. Other corrosion inhibitors include zinc nitrate, magnesium nitrate, and sodium nitrate.
The foregoing ingredients are exemplary of the various ingredients and types of ingredients that may be employed, and it is contemplated that other ingredients or other examples of the categories of dampening solution ingredients besides those given above may be included.
In use, the dampening solution may be applied to any suitable lithographic printing plate, such as a photosensitive lithographic printing plate; a deep-edge plate; a multi-layer metallic plate, such as a bi-metal or tri-metal plate; a direct-drawing master, or a lithographic printing plate for electrophotography. Generally, the lithography process will include the steps of providing a lithographic plate, dampening the plate, applying ink to the plate, transferring the inked image to a rubber blanket, and transferring the rubber blanket image to a substrate. The dampening solution may be used in connection with other lithographic processes as may be known or otherwise found to be suitable, such as image erasing and plate protection.
The ability of corn hull hemicellulose to function in a dampening solution is surprising, given the marked variation in composition between corn hull hemicellulose and gum arabic. The following table provides a summary of typical components of corn hull hemicellulose and gum arabic, as well as the makeup of three of the soybean hemicellulose products, as reported in U.S. Pat. No. 5,615,613.
Moreover, corn hull hemicellulose is more readily obtainable than soybean hemicellulose. Corn seeds are composed of about 5.3% hulls, and the potential yield of hemicellulose from corn hulls is about 55%. Soybean hulls are obtainable from okara, which is the insoluble reside obtained in the process of manufacturing soy milk (see Moizudding et al., J. Food Sci, 64: 145-48 (1999)). As is evident from data in the Examples, the yield of corn hull hemicellulose is three to four times the yield of soybean hemicellulose from okara.
The following Examples are provided to illustrate the present invention, but should not be construed as limiting the invention in scope.